Various electro-optical readers have previously been developed for reading bar code symbols appearing on a label, or on a surface of a target. The bar code symbol itself is a coded pattern of indicia. Generally, the readers electro-optically transform graphic indicia of the symbols into electrical signals, which are decoded into alphanumeric characters. The resulting characters describe the target and/or some characteristic of the target with which the symbol is associated. Such characters typically comprise input data to a data processing system for applications in point-of-sale processing, inventory control, article tracking and the like.
The specific arrangement of symbol elements, e.g., bars and spaces, in a symbol defines the characters represented according to a set of rules and definitions specified by a code or symbology. The relative size of the bars and spaces is determined by the type of code used, as is the actual size or widths of the bars and spaces, as measured between opposite edges of each element.
Electro-optical readers have been disclosed, for example, in U.S. Pat. No. 4,251,798; U.S. Pat. No. 4,369,361; U.S. Pat. No. 4,387,297; U.S. Pat. No. 4,409,470; U.S. Pat. No. 4,760,248; and U.S. Pat. No. 4,896,026, and generally include a light source consisting of a gas laser or semiconductor laser for emitting a laser beam. The laser beam is optically modified, typically by a focusing optical assembly, to form a beam spot having a certain size or cross-section at a predetermined target location or beam waist. Preferably, the cross-section of the beam spot at the target location approximates the minimum width between symbol regions of different light reflectivity, i.e., the bars and spaces, although the cross-section of the beam spot can be at least three times larger than the minimum width between the symbol regions.
In conventional readers, the laser beam is directed by a scanning component along an outgoing optical path toward a target symbol for reflection therefrom. The reader operates by repetitively scanning the laser beam in a scan pattern, for example, a line or a series of lines across the target symbol by movement of the scanning component, such as a scan mirror, disposed in the path of the laser beam. The scanning component may sweep the beam spot across the symbol, trace a scan line across and beyond the boundaries of the symbol, and/or scan a predetermined field of view.
Readers also include a photodetector, which functions to detect laser light reflected or scattered from the symbol. In some systems, the photodetector is positioned in the reader in a return path so that it has a field of view, which extends at least across and slightly beyond the boundaries of the symbol. A portion of the laser beam reflected from the symbol is detected and converted into an analog electrical signal. A digitizer digitizes the analog signal. The digitized signal from the digitizer is then decoded by a decoder, based upon the specific symbology used for the symbol, into a binary data representation of the data encoded in the symbol. The binary data may then be subsequently converted into the alphanumeric characters represented by the symbol.
Many applications call for a hand-held reader where a user aims the laser beam at the symbol, and the beam executes a scan pattern to read the symbol. For such applications, the arrangement of electro-optical components must be compact in order to be accommodated in a hand-held package, which may be pistol-shaped. Moreover, such readers must be lightweight and structurally robust to withstand physical shock resulting from rough handling. It is also desirable that minimal power be consumed during operation to promote on-board battery usage.
Overall performance of the reader for reading symbols is a function of the optical components which direct the laser beam at the target symbol and which collect the reflected light, and a function of the electronic components which convert and process the information contained in the reflected light. A measure of the overall performance of the reader is its ability to resolve the narrowest elements of the symbol and its ability to successfully decode symbols located both close-up and far-away from the reader, also known as the working range.
The scan pattern over the symbol can take a variety of forms, such as a repeated line scan, a standard raster scan, a jittered raster scan, a fishbone scan, a petal scan, etc. These patterns are generated by controlled motions of the scan mirror oscillated by some form of motor drive to periodically deflect the beam through the desired pattern. For a repeated beam pattern, a polygonal mirror is unidirectionally rotated by a simple motor. The more times a symbol can be scanned in a given time period, the greater the chances of obtaining a valid read of the symbol.
It is known, for example, in U.S. Pat. No. 6,155,490; U.S. Pat. No. 6,616,046; and U.S. Pat. No. 7,007,843 to use microelectromechanical systems (MEMS) technology to eliminate macroscopic mechanical and electronic components and to replace them with miniature scan elements or mirrors to sweep the laser beam across the indicia to be electro-optically read. These MEMS-based systems are generally fabricated using integrated circuit fabrication techniques or similar techniques such as surface micromachining or bulk micromachining. A common material used is polycrystalline silicon (polysilicon).
Nevertheless, there are drawbacks concerning current implementations of readers utilizing MEMS scan mirrors. The MEMS-based reader is constrained to use a staring collection system, i.e., a non-retro-collection system, in which the photodetector faces or stares at the indicia. To collect sufficient light, the collection system must have a relatively large light detection area. However, a large light detection area for the collection system tends to be more sensitive to ambient light because the collection system will have a relatively larger field of view, which must be large enough to see the entire scan line, as opposed to retro-collection systems where the field of view needs only be large enough to see a small area around the moving beam spot. Ambient light, however, creates noise that limits the working range.
Also, current implementations of the collection systems of MEMS-based readers tend to produce analog electrical signals from the photodetectors of relatively poor signal quality as compared, for example, to retro-collective systems. The lack of adequate laser power safeguards forces some readers to emit laser beams of lower output power which, in turn, decreases the magnitude of the analog electrical signals from the photodetectors. These factors also contribute to limiting the working range.